linux-kernel - Re: [PATCH 4/7] ptrace: Partly fix set_task_blockstep()->update

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Date:	Mon, 10 Sep 2012 19:45:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland McGrath <roland@...k.frob.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] ptrace: Partly fix
 set_task_blockstep()->update_debugctlmsr() logic

On Mon, 2012-09-10 at 18:57 +0200, Sebastian Andrzej Siewior wrote:
> The only user that is touching this bits in irq context is perf. perf
> uses raw_local_irqsave() (raw_* most likely due to -RT).

# git grep raw_local_irq arch/x86/kernel/cpu/perf_* kernel/events/ | wc -l
0

I think you're confusing raw_spin_lock_irq{,save}() with
raw_local_irq{,save}(), those two are not the same.

raw_spin_lock* is a lock that is always a spinlock -- unlike spin_lock,
which can be a PI-mutex (PREEMPT_RT=y).

raw_local_irq* is like arch_spin_lock* and avoids things like tracing
and lockdep and should not be used unless you really know wth you're
doing, and then still not ;-)

>  I have no idea
> what you can against NMI unless not touching the register in NMI
> context. 

I think perf will actually touch this from NMI context under 'rare'
conditions, in particular:

  x86_pmu_handle_irq() (NMI handler)
    x86_pmu_stop()
      x86_pmu.disable -> intel_pmu_disable_event()
        intel_pmu_lbr_disable()
          __intel_pmu_lbr_disable()
            wrmsrl(MSR_IA32_DEBUGCTLMSR,... );

I suppose I should look into it not doing that...
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